Among the genetic alterations that have been shown to represent direct or indirect causative agents of proliferative diseases, such as cancers, there may be cited mutations occurring at loci harboring genes that are called tumor suppressor genes.
Tumor suppressor genes are defined as genes involved in the control of abnormal cell proliferation and whose loss or inactivation is associated with the development of malignancy. Tumor suppressor genes encompass ortho-genes, emerogenes, flatogenes, and onco-suppressor genes.
More specifically, tumor suppressor genes are genes whose products inhibit cell growth. Mutant alleles in cancer cells have lost their normal function, and act in the cell in a recessive way in that both copies of the gene must be inactivated in order to change the cell phenotype. The tumor phenotype can be rescued by the wild-type allele, as shown by cell fusion experiments first described by Harris and colleagues (Harris H. et al., 1969). Gemmline mutations of tumor suppressor genes may be transmitted and thus studied in both constitutional and tumor DNA from familial or sporadic cases. The current family of tumor suppressors include DNA-binding transcription factors (i.e. p53, WT1), transcription regulators (i.e., RB, APC) and protein kinase inhibitors (i.e. p16).
The existence of tumor suppressor genes has been particularly shown in cases of hereditary cancers. These are cancer where there is a clear pattern of inheritance, usually autosomal dominant, with a tendency for earlier age of onset than for sporadic tumors.
Tumor suppressor genes are detected in the form of inactivating mutations that are tumorigenic. The two best characterized genes of this class code for the proteins RB (Retinoblastoma protein) and p53.
Retinoblastoma is a human childhood disease, involving a tumor in the retina. It occurs both as an inheritable trait and sporadically (by somatic mutation). Retinoblastoma arises when both copies of the RB gene are inactivated. In the inherited form of the disease, one parental chromosome carries an alteration in this region, usually a deletion. A somatic event in retinal cells that causes the loss of the other copy of the RB gene causes a tumor. Forty percent of cases are hereditary, transmitted as an autosomal dominant trait with 90% penetrance. Of these cases, around 10-15% are transmitted from an affected parent, the remaining arising as de novo germ-line mutations. In the sporadic form of the disease, the parental chromosomes are normal, and both RB alleles are lost by somatic events. The tumor suppressor nature of RB was shown by the introduction of a single copy of RB1 into tumor cell lines lacking the gene, resulting in complete or partial suppression of the tumorigenic phenotype.
The RB protein has a regulatory role in cell proliferation, acting via transcription factors to prevent the transcriptional activation of a variety of genes, the products of which are required for the onset of DNA synthesis, the S phase of the cell cycle.
When investigating on the molecular function of RB, it has been found that the RB protein interacts with a variety of viral proteins, including several tumor antigens, such as SV40 T antigen, adenovirus E1A protein, human papillomavirus E7. These viral proteins have been shown to bind to RB, thereby inactivating it and allowing cell division to occur.
Thus, an important step toward defining a mechanism underlying tumor suppressor activity of the RB gene was the observation that the transforming products of adenovirus (E1A protein), simian virus 40 (large T antigen) and human papillomavirus (E7 protein) could precipitate wild-type RB protein. This, in turn, led to the identification of a family of cellular proteins that can reversibly bind to a discrete domain on the RB protein, referred to as the T/E1A pocket by using the same specificity as the viral products. The subsequent observation that protein binding was inhibited following RB protein phosphorylation in the late G1 phase of the cell cycle suggested the hypothesis that the RB protein, as well as the related product p107, may regulate the functional activity of its binding partners by a cell-cycle dependent pattern of physical association. In particular, the activity of the RB protein has been shown to be regulated through cell cycle-dependent phosphorylation by cyclin-dependent kinases.
The picture of transcription regulation is made even more complex by the finding that a number of RB related proteins (e.g. p107 and p130) also bind members of the E2F family and are therefore involved in regulatory process.
In view of the foregoing, there clearly exists a pressing need to identify and characterize the cellular proteins that interact with the retinoblastoma protein in order to provide diagnostic and therapeutic tools useful to prevent and cure cell differentiation disorders, particularly disorders in which a lack of completion of cell differentiation, particularly in terminal cell differentiation, or in which an abnormal cell proliferation is detected, such as in proliferative diseases like cancer.
For the purpose of the present invention, cells with abnormal proliferation include, but are not limited to, cells characteristic of the following disease states: thyroid hyperplasia, psoriasis, benign prostatic hypertrophy, cancers including breast cancer, sarcomas and other neoplasms, bladder cancer, colon cancer, lung cancer, prostate cancer, various leukemias and lymphomas.